1. Field of the Invention
The present invention relates to a method for producing an oxygen-free renewable fuel in a higher yield via hydrogenation and hydrodeoxygenation reactions of an oil or fat in a supercritical fluid at a lower hydrogen pressure and a lower reaction temperature than conventional methods for producing renewable fuels.
2. Description of the Related Art
In recent years, energy resource depletion and environmental pollution problems associated with the excessive use of fossil fuels have led to the widespread utilization of renewable, sustainable and highly environmentally friendly fuels based on non-fossil fuel sources. Biodiesel, a kind of biofuel, is considered as the most practical new renewable energy source and has been investigated worldwide for its use as a direct fuel or an additive.
A typical biodiesel production method is depicted in Reaction Scheme 1:

That is, transesterification of a triglyceride present in a vegetable oil or fat, an animal oil or fat or waste cooking oil with an alcohol in the presence of an acid or alkali catalyst affords a fatty acid methyl ester (FAME). For example, one triglyceride molecule reacts with three methanol molecules to produce three FAME molecules and one glycerol molecule. The FAME is used as biodiesel that substitutes for fossil fuel-based diesel.
FAME biodiesel has an advantage in that it can be applied to engine structures and mechanisms of current diesel fueled vehicles. However, the presence of ester groups containing oxygen atoms and double bonds in the FAME molecules often causes stability problems during long-term storage, such as sludge and sediment formation. Further, FAMEs cause softening, swelling, hardening and cracking of rubbers which are used as seals of engine parts and fuel supply systems due to their intrinsic molecular properties, resulting in fuel leakage during long-term use. The presence of oxygen atoms makes FAMEs highly soluble in water. When FAME biodiesel is applied to diesel engine vehicles, free fatty acids can cause corrosion of the engine systems including metal parts, such as internal control units and fuel injection nozzles. In addition, FAMEs are known to produce higher nitrogen oxide (NOx) emissions due to the presence of oxygen atoms therein when compared to typical fossil fuel-based diesel.
The above mentioned problems of FAMEs are caused by the formation of difficult-to-separate free fatty acids in FAME-based biodiesel and the presence of oxygen atoms in the FAME molecules. Accordingly, it is believed that the problems at issue in current FAME biodiesel can be overcome by the production of renewable fuels that contain no oxygen while producing the same molecular formula as existing fossil fuel-based fuels.
In view of this, many proposals have been made on methods for producing oxygen-free hydrocarbon-based materials from oils and fats. An exemplary method is depicted in Reaction Scheme 2:

This method involves hydrogenation and hydrodeoxygenation to saturate the double bonds present in the triglycerides with hydrogen to single bonds in the presence of a suitable catalyst, followed by three major reaction pathways, i.e. decarboxylation, decarboxylation or hydrodeoxygenation, to produce an oxygen-free renewable fuel.
U.S. Pat. No. 4,300,009 suggests a process for manufacturing hydrocarbon-based compounds suitable for use as fuels of gasoline engines via hydrodeoxygenation and cracking reactions of corn oil, castor oil or tall oil as a raw material in the presence of highly crystalline zeolite as a catalyst. Further, U.S. Pat. No. 4,992,605 suggests a process for producing C15-C17 paraffins suitable for use as fuels of diesel engines via hydrodeoxygenation of canola oil, sunflower oil or rapeseed oil as a raw material in the presence of cobalt-molybdenum (Co—Mo) as a catalyst. Further, U.S. Pat. No. 5,705,722 suggests a process for producing a material suitable for use as a diesel fuel cetane number improver via hydrodeoxygenation of relatively inexpensive oils and fats, such as tall oil, waste cooking oil and animal oils and fats, as raw materials in the presence of nickel-molybdenum (Ni—Mo)-supported alumina as a catalyst. According to the these patents, however, the reactions require high temperatures between 350 to 450° C. and high hydrogen pressures between 100 to 200 bar, and the catalysts are susceptible to coking during reactions for a long time, resulting in low yields of the products.
Hydrogenation and hydrodeoxygenation reactions are very exothermic and release a large amount of heat. It is very important to control the reaction temperatures because the amount of heat released varies depending on the kind of oils and fats with different numbers of double bonds. For example, one to five double bonds may be present in one triglyceride molecule. Accordingly, there is a difficulty in controlling the amount of heat released during the reactions. High temperature causes the occurrence of side reactions, such as cracking and aromatization reactions, other than the required hydrodeoxygenation, leading to a low yield of renewable fuels. The side reactions leave excess impurities that negatively affect the characteristics of renewable fuels and that cause coking of catalysts during long-term operation to shorten the life of the catalysts. In an attempt to prevent the occurrence of side reactions, a low reaction temperature is considered. However, a low conversion of triglycerides is a problem at the low reaction temperature.
Very high hydrogen pressures of at least 100 bar are required for the production of renewable fuels. Particularly, higher hydrogen pressures of at least 150 bar are necessary to facilitate reactions for the production of renewable fuels in higher yield because of low solubility of hydrogen in an oil or fat. For example, a very small amount (4 to 6 g) of hydrogen is dissolved in 100 g of an oil or fat at room temperature. The hydrogen solubility in an oil and fat does not increase significantly despite an increase in temperature. Accordingly, hydrogenation and hydrodeoxygenation reaction rates are determined by a mass transfer process in which gaseous hydrogen is dissolved in a liquid oil or fat. Rapid dissolution of a large amount of hydrogen in an oil or fat as a reactant increases the possibility and frequency of contact of the hydrogen with the catalyst surface. This activates the catalytic reactions, leading to high yield. If the amount of hydrogen dissolved in an oil or fat is not sufficient, the reaction rates decrease and cokes are readily formed on the catalyst surface. The coking promotes deactivation of the catalyst, leading to a low yield of a renewable fuel. Thus, there is a need to use hydrogen in a larger amount than is necessary for hydrogenation and hydrodeoxygenation reactions. Consequently, considerable equipment and operating costs are required to maintain high temperature and pressure conditions. Further costs are incurred to ensure safety against explosion of high-pressure hydrogen.